The in vitro interaction of alveolar macrophages with Pneumocystis carinii will be studied. Macrophage monolayers cultivated in vitro will be challenged with Pneumocystis carinii suspensions derived from glucocorticoid treated rats and the fate of ingested organisms monitored and the effects of macrophage maturation, activation, and in vivo steroid treatment on macrophage function will be assessed. In addition, requirements for non-opsonized phagocytosis of Pneumocystis carinii and other particles by alveolar macrophages will be defined, especially as related to in vitro maturation and to soluble factors which may be produced by macrophages. Alveolar macrophages will be obtained from healthy and steroid treated animals and also from animals immunized specifically. Parameters which enhance uptake by freshly explanted macrophages and those which inhibit uptake by macrophages which have matured in vitro will be studied and the specificity of the interactions defined.